This invention relates to an AC capacitor having improved AC life and corona characteristics, and more particularly to a dielectric film aluminum foil capacitor or a metallized film capacitor having at least the ends thereof coated with a urethane containing unreacted isocyanate groups.
Protective or potting layers have been used under hardenable resin encapsulants. For example, silicone elastomers have been used as protective coatings on dry capacitors under a hardenable resin encapsulant to prevent mechanical stress of the capacitor dielectric during the curing of the encapsulating resin. Likewise, electrical components have been potted in polyesters and ceramic capacitors in resilient elastomers to protect the units if the outer case cracks because of differential thermal expansion of the units and the encapsulating materials.
Film-foil or metallized film capacitors, when encapsulated in rigid insulating materials such as epoxies, may perform very well in DC applications and yet fail in a relatively short time in AC applications at equivalent voltage stress. Among the capacitors having plastic film dielectrics, e.g., polypropylene, polycarbonate and polyester, this problem is most obvious with polypropylene which is largely used in AC applications to take advantage of its very low dissipation factor.